This is my nephews truck, and he's telling me it seems to be fairly random. It's done it once for me, my first hand account is as follows:

Drove the truck 10 miles or so in the morning, no issues at all. An hour or two later I drove it about 3 miles, no issues. An hour or so later I take off and within about 100' its hesitating bad and idling rough. I quickly popped the hood and checked the fuel pressure gauge I left hooked up. Still holding 50 psi. Took off again (stop and go in town) and all was well. No issues driving to work this morning.



He's told me sometimes it'll backfire (pop under the hood), and turning it off, letting it sit a minute, and starting back seems to fix it until it randomly happens again.

My guess is something like an ignition module (that's where I'd be looking on my Chevy anyway)? Plugs, wires, cap & rotor are less than a year and 4,000 miles old. Just ran a can of sea foam through it. So far the fuel pressure always holds 50 psi.

The truck also has 3 new scuffs/dents in the last year and he's driving mine until I fix his, so obviously I need to figure this one out quickly!

Any thoughts or words of wisdom?

Thanks,

Mike

 

Update:

Just made a few short trips around town. The problem showed itself 2 of 3 trips, there is some backfiring, especially when it's hesitating and you give it extra fuel...which again leads me to believe something in the ignition control circuit.

 

Had a similiar issue. Problem wound up being a bad crankshaft sensor located behind the right front wheel well in the bell housing. Unfortunately on my truck which is a 98 3.9L, this is the hardest sensor to get too. I replaced all the sensors on the truck because of age and I got a bad crank sensor....defective. Cam sensor locate in the distributor can do the same thing since they work together controlling timing. Popping under the hood was a symptom, erattic misfire, sometimes would run just fine. Sometimes it would just stall for no reason and start right back up. I thought timing chain because of the miles but after a check turning the crank and watching the distributor rotor there was no lag at all thus leading me back to the sensors.....hope it helps!

 

As I've driven it more I've noticed that I can back way off the throttle, to idle or close, and it seems to run smooth. Give it some more fuel and there it goes again hesitating, spitting, & sputtering.

Did yours do this?

I would think when a cam or crank sensor is acting up it would still misfire at idle or little throttle?

so far TPS, MAP, intake air, and CTS sensors all resistance test to spec...although the problem comes and goes so I suppose the culprit could still test good a time or two.

 

Replaced the crank sensor today (what a pain), no change.

Sunday I put an analog volt meter in the cab and ran wires to the engine compartment where I back probed the Cam position (aka distributor pickup) sensor, TPS, and MAP sensor in order to see what they were reporting when the engine started acting up. No real difference between normal operation and spit & sputtering beyond what would be expected - such as frequency of cam sensor pulses slowing as engine speed slows. I even ran a few miles with the MAP sensor unplugged which made things slightly worse.

The short of it: I'm stumped. Anyone have ideas?

 

look at this it could be related

https://dodgeforum.com/forum/2nd-gen...um-thread.html

 

Have you tried wiggling the middle connector on the PCM? My 98 is having a similar issue and it acts up when i wiggle the middle connector which ive read is a sign of the PCM going bad.

 

Not yet but thanks for the heads up.

Before I solved it a gas tank strap broke. Off came the bed, then I discovered the place on the frame I was worried about was much worse than I thought.

Frame is painted, new straps are on, soon as I get the bed back on (hopefully tomorrow) I'll get back to troubleshooting...

 

About to jump back in to troubleshooting. I did wiggle the middle PCM connector and no change.

Im going to swap 6 of the plug wires off my truck to make sure they're good. I was going to replace plugs too (plugs & wires only have 4,000 miles or so on them). I did a search on a couple parts store sites and all were coming up with platinum & iridium plugs. Regular plugs are in it.

GM TBI's are funny about not liking anything but regular AC Delcos. Do these MPI 3.9's have a preference?

 

I'm a new member but I've had this problem. My beloved '99 3.9 did the same thing. Went round and round with it, mechanics were stumped, had the truck for 3 weeks. I replaced everything from crank sensor to throttle body. Hesitation, TB backfires, went away, came back, etc..

Turns out one of the mechanics was under the hood while it was doing it and dropped the trouble light which broke. That's when he noticed that there was an intermittent spark at the base of the distributor where it clamps down. Replaced and we were in business. Can't see the sparks unless you're right up on it too.